A hollow fiber membrane module has been widely used, for instance, for membrane filtration treatments in the production process of various industrial products such as in the production of ultrapure water for washing semiconductors or pyrogen free water; in a closed process wherein electrodeposition paints are collected; enzyme condensation; bacteria removal in fermentation liquid; and removal of turbidity from pure water, waste water or the like. Among the above, in the water treatment field such as treatments for tap water and waste water, a larger scale hollow fiber membrane module has recently been demanded to reduce the treatment costs.
Heretofore, in the hollow fiber membrane modules as described above, epoxy resin has been used as a potting material for bonding and fixing hollow fiber membranes and a module housing. However, there has been a problem that high elasticity of the epoxy resin causes easy breakage of the hollow fiber membranes at the bonding interface with the epoxy resin. Therefore, the technique of preventing breakage of the hollow fiber membrane around the interface by filling the bonding interface between epoxy resin and hollow fiber membranes with flexible resins such as silicone rubber has been adopted. However, this process has the drawback in that the bonding step had to be conducted twice, resulting in inferior economical efficiency. On the other hand, the use of urethane resin as a potting material has been recently proposed (e.g. JP-A-7-47239 and JP-A-7-148421). For example, JP-A-7-47239 discloses that urethane resin prepared by curing prepolymers having an isocyanate end group obtained from diphenyl methane diisocyanate and polyoxy tetramethylene glycol (hereinafter referred to as PTMG) and a curing agent comprising PTMG and castor oil or derivative polyol of castor oil has a higher side temperature in the rubber region of more than 100° C. and a storage modulus of the level of 107 N/m2 and a hollow fiber membrane module obtained by using this urethane resin as a potting material can stand continuous filtration of hot water of 90° C. under a differential pressure of 0.2 MPa (200 KPa) for 6 months without leakage.
However, the urethane resin constructed as the above can hardly achieve storage modulus of 108 N/m2 or more. As a result, there are problems, in that potting portion considerably transforms by different pressure and leakage easily occurs when it is adopted for a hollow fiber membrane module having a large diameter.
Furthermore, JP-A-7-148421 discloses the technique for reducing the transformation of potting portions by using a unit filtration element comprising a hollow fiber membrane bundle installed in a fluid-permeable protecting cylindrical tube and a beam having a length extending from the potting portion at one end of the case housing to the potting portion at the other end of the case housing. However, this module construction has the problems of a complex manufacturing process and high costs because of the material for producing the unit filtration element and the beam material and of the processes for assembling these materials.
WO 97/10893 discloses, as a means for preventing transformation of the potting portion and improving durability, a rib fixed directly to the inner wall of housing in the state of being embedded in the potting portion. This document also discloses fixing by welding, fixing by combination of notches, fixing by screwing and integral molding as a means for fixing rib directly to a housing. Moreover, the technology for preventing damage of potting portion or improving the seal property is also disclosed in JP-A-63-171606, JP-A-6-296834 and JP-A-11-300173.